Recently some systems for providing a variety of services taking advantage of radio communication have been commercialized. In Japan for instance, the following systems are in the actual use: GPS (global positioning system) which uses satellites to measure a distance, VICS (vehicle information and communication system) which provides road-traffic information, and ETC (electronic toll collection) system which collects automatically tolls of highway. A mobile radio apparatus such as a vehicle-mounted apparatus desirably includes an antenna that can handle a plurality of frequency bands corresponding to the foregoing systems. Thus a multiple antenna incorporating individual frequency bands is demanded. A conventional multiple antenna is described hereinafter with reference to FIG. 5 and FIG. 6.
FIG. 5 shows a perspective view of the conventional multiple antenna. As shown in FIG. 5, dielectric substrate 52 made of dielectric material is prepared on the top surface of planar ground electrode 51 made of copper. On top of dielectric substrate 52, planar antenna electrodes 53a and 53b made of copper are placed in parallel with each other. Feeding points 54a and 54b of antenna electrodes 53a and 53b are respectively coupled with feeding terminals (not shown). Conventional multiple antenna 500 is thus constructed. In this structure, antenna electrode 53a transmits and receives signals of frequency band f1, and antenna electrode 53b transmits and receives signals of frequency band f2.
FIG. 6 shows a perspective view of another conventional multiple antenna, which was designed in response to the request of downsizing. In this multiple antenna, first dielectric substrate 62a equipped with first antenna electrode 63a is placed on planar ground electrode 61. First planar antenna 601 is thus constructed. On top of first planar antenna 601, second dielectric substrate 62b equipped with second antenna electrode 63b is placed. Second planar antenna 602 is thus constructed. Feeding terminals 65a, 65b extending through ground electrode 61 are respectively coupled to feeding points 64a, 64b of antenna electrodes 63a, 63b. The another conventional multiple antenna 600 is thus constructed.
In the foregoing structure, first planar antenna 601 transmits and receives signals of frequency band f1, and second planar antenna 602 transmits and receives signals of frequency band f2.
On top of second planar antenna 602, a third planar antenna is placed for transmitting and receiving signals of frequency band f3. Such a structure allows transmitting and receiving signals of three or more than three frequency bands.
The prior art discussed above is disclosed, e.g. in Japanese Patent Examined Publication No. 2002-26634.
However, since conventional multiple antenna 500 has a plurality of planar antennas, and they are placed in parallel with each other on the flat face of dielectric substrate 52, the external shape becomes so bulky that antenna 500 is unfit for being downsized.
On the other hand, another conventional multiple antenna 600, which was designed to be smaller size, has a plurality of planar antennas piled up one on another. Those planer antennas thus interfere with each other between upper one and lower one, so that the radiation efficiency lowers, and it is difficult to raise the radiation efficiency over 50%. Meanwhile the radiation efficiency is a ratio of a magnitude of an output signal vs. a magnitude of an input signal.
Multiple antenna 600 becomes higher as the number of frequency bands to be transmitted and received increases, so that antenna 600 is unfit for lowering the profile if the number of applicable frequency bands increases.